1. Field Of The Invention
The present invention relates generally to devices for measuring fluid flow speed using differential pressure, specifically to devices measuring the speed of devices or vessels, and more specifically to devices for measuring the speed of low speed devices or vessels.
2. Description Of The Related Art
A commonly used device for the measurement of either air or water speed of vehicles is the pitot tube. A description of how the pitot tube operates as well as specific examples of other speed measurement devices that operate by sensing fluid pressure are set forth in U.S. Pat. No. 6,308,581, which is herein incorporated by reference.
The invention disclosed in U.S. Pat. No. 6,308,581 is a differential pressure flow sensor that alleviates many of the problems associated with previous speed measurement devices such as protrusion of the speed measurement device from the surface of the vessel directly into the fluid flow and wear and friction issues due to moving parts. The invention overcame these previous problems by providing a differential pressure flow meter that includes a longitudinal recess angled to the surface of the vessel so as to form a ramp, and a dynamic port surface angled extending from the floor of the ramp to the surface. A dynamic port is formed in the dynamic port surface and is connected through a dynamic port channel to a pressure transponder. Optionally, a static port, which is basically a port that is normal to the fluid flow, may be provided in the ramp or from the surface of the vessel. The static port may be connected through a static port channel to the pressure transponder which is used to determine the differential pressure between the static and dynamic ports. The pressure in the dynamic port channel or differential pressure between the dynamic port and the static port channels is related to the flow rate of fluid across the surface of the vessel.
The above described differential pressure flow sensor operates far more precisely than previous prior art measuring devices for vehicles travelling at high speeds. However, for vehicles travelling at very low speeds (below 5 knots), the above sensor may be less accurate due to the smaller change in differential pressure between the dynamic and static port channels as well as vessel heaving causing dynamic flow into the static port (which is insignificant at higher speeds).
Therefore, a differential pressure flow sensor is desired that performs more accurately at lower speeds.
The invention consists of an improvement to the invention disclosed in U.S. Pat. No. 6,308,581 to improve the accuracy of measurements at low speeds. The improvement uses the concept of forgoing depth measurement using a static port to increase differential pressure through use of a low pressure port, at an angle less than 90 degrees to the dynamic port, wherein fluid flow draws pressure from the low pressure port in a direction counter to fluid flow entering the dynamic port. By using this concept, the device accuracy is improved at low speeds, however, for higher speed applications, the prior art device may still provide more accurate measurements.
Accordingly, it is an object of this invention to provide an improved fluid flow sensor for low speed applications.
It is a further object of this invention to provide an improved fluid flow sensor for flow speedometer on a vessel for low speed applications.
It is a still further object of this invention to provide a higher output differential flow pressure flow sensor for low speed applications.
It is yet a further object of this invention to provide a more precise flow sensor for low speed devices or vessels.
This invention accomplishes these objectives and other needs related to measuring fluid flow speed using differential pressure by providing an improvement upon the differential pressure flow sensor of U.S. Pat. No. 6,308,581 that provides a larger pressure differential for a given flow, and, thereby allows a more precise measurement of flow for low speed devices or vessels. The improved differential pressure flow sensor comprises an improvement to the prior art differential pressure fluid flow meter described in U.S. Pat. No. 6,308,581 for low speed applications. The prior art fluid flow meter includes a longitudinal recess angled to the surface of the vessel so as to form a ramp, and a dynamic port surface angled extending from the floor of the ramp to the surface. The dynamic port is formed in the dynamic port surface and is connected through a dynamic port channel to a pressure transponder. The improvement comprises combining the above described differential pressure fluid flow meter with a low pressure port, at an angle less than 90 degrees to the dynamic port, wherein fluid flow draws pressure from the low pressure port in a direction counter to fluid flow entering the dynamic port.